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          <p>「这是我参与2022首次更文挑战的第9天，活动详情查看：<a target="_blank" rel="noopener" href="https://juejin.cn/post/7052884569032392740" title="https://juejin.cn/post/7052884569032392740">2022首次更文挑战</a>」</p>
<h2 id="使用hooks代替redux？"><a href="#使用hooks代替redux？" class="headerlink" title="使用hooks代替redux？"></a>使用hooks代替redux？</h2><p>在react hooks刚出来出来的时候，有很多人都觉得完全可以用hooks代替调繁琐的redux。比如下面这段代码 ：</p>
<figure class="highlight jsx"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br><span class="line">60</span><br><span class="line">61</span><br><span class="line">62</span><br><span class="line">63</span><br><span class="line">64</span><br><span class="line">65</span><br><span class="line">66</span><br><span class="line">67</span><br><span class="line">68</span><br><span class="line">69</span><br><span class="line">70</span><br><span class="line">71</span><br><span class="line">72</span><br><span class="line">73</span><br><span class="line">74</span><br><span class="line">75</span><br><span class="line">76</span><br><span class="line">77</span><br><span class="line">78</span><br><span class="line">79</span><br><span class="line">80</span><br><span class="line">81</span><br><span class="line">82</span><br><span class="line">83</span><br><span class="line">84</span><br><span class="line">85</span><br><span class="line">86</span><br><span class="line">87</span><br><span class="line">88</span><br><span class="line">89</span><br><span class="line">90</span><br><span class="line">91</span><br><span class="line">92</span><br><span class="line">93</span><br><span class="line">94</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 创建context</span></span><br><span class="line"><span class="keyword">const</span> <span class="title class_">ThemeContext</span> = <span class="title class_">React</span>.<span class="title function_">createContext</span>(<span class="literal">null</span>);</span><br><span class="line"><span class="comment">// 初始state</span></span><br><span class="line"><span class="keyword">const</span> initState = &#123;</span><br><span class="line">  <span class="attr">primary</span>: <span class="string">&#x27;blue&#x27;</span>,</span><br><span class="line">  <span class="attr">secondary</span>: <span class="string">&#x27;gray&#x27;</span>,</span><br><span class="line">  <span class="attr">fontSize</span>: <span class="string">&#x27;20px&#x27;</span>,</span><br><span class="line">  <span class="attr">bg</span>: <span class="string">&#x27;aliceblue&#x27;</span>,</span><br><span class="line">&#125;;</span><br><span class="line"><span class="comment">// 创建reducer</span></span><br><span class="line"><span class="keyword">const</span> <span class="title function_">reducer</span> = (<span class="params">state, action</span>) =&gt; &#123;</span><br><span class="line">  <span class="keyword">switch</span> (action.<span class="property">type</span>) &#123;</span><br><span class="line">    <span class="keyword">case</span> <span class="string">&#x27;CHANGE_COLOR&#x27;</span>:</span><br><span class="line">      <span class="keyword">return</span> &#123; ...state, ...action.<span class="property">payload</span> &#125;;</span><br><span class="line">    <span class="keyword">case</span> <span class="string">&#x27;CHANGE_BG&#x27;</span>:</span><br><span class="line">      <span class="keyword">return</span> &#123; ...state, <span class="attr">bg</span>: action.<span class="property">payload</span> &#125;;</span><br><span class="line">    <span class="keyword">case</span> <span class="string">&#x27;CHANGE_SIZE&#x27;</span>:</span><br><span class="line">      <span class="keyword">return</span> &#123; ...state, <span class="attr">fontSize</span>: <span class="string">&#x27;30px&#x27;</span> &#125;;</span><br><span class="line">    <span class="attr">default</span>:</span><br><span class="line">      <span class="keyword">return</span> state;</span><br><span class="line">  &#125;</span><br><span class="line">&#125;;</span><br><span class="line"></span><br><span class="line"><span class="keyword">function</span> <span class="title function_">App</span>(<span class="params"></span>) &#123;</span><br><span class="line">  <span class="keyword">const</span> [theme, dispatch] = <span class="title function_">useReducer</span>(reducer, initState);</span><br><span class="line">  <span class="keyword">const</span> changeBg = <span class="title function_">useCallback</span>(<span class="function">(<span class="params">bg</span>) =&gt;</span> &#123;</span><br><span class="line">    <span class="title function_">dispatch</span>(&#123; <span class="attr">type</span>: <span class="string">&#x27;CHANGE_BG&#x27;</span>, <span class="attr">payload</span>: bg &#125;);</span><br><span class="line">  &#125;, []);</span><br><span class="line">  <span class="keyword">const</span> changeColor = <span class="title function_">useCallback</span>(<span class="function">() =&gt;</span> &#123;</span><br><span class="line">    <span class="title function_">dispatch</span>(&#123;</span><br><span class="line">      <span class="attr">type</span>: <span class="string">&#x27;CHANGE_COLOR&#x27;</span>,</span><br><span class="line">      <span class="attr">payload</span>: &#123; <span class="attr">primary</span>: <span class="string">&#x27;darkgreen&#x27;</span>, <span class="attr">secondary</span>: <span class="string">&#x27;burlywood&#x27;</span> &#125;,</span><br><span class="line">    &#125;);</span><br><span class="line">  &#125;, []);</span><br><span class="line">  <span class="keyword">const</span> changeSize = <span class="title function_">useCallback</span>(<span class="function">() =&gt;</span> &#123;</span><br><span class="line">    <span class="title function_">dispatch</span>(&#123; <span class="attr">type</span>: <span class="string">&#x27;CHANGE_SIZE&#x27;</span> &#125;);</span><br><span class="line">  &#125;, []);</span><br><span class="line">  <span class="keyword">return</span> (</span><br><span class="line">    <span class="language-xml"><span class="tag">&lt;<span class="name">ThemeContext.Provider</span></span></span></span><br><span class="line"><span class="tag"><span class="language-xml">      <span class="attr">value</span>=<span class="string">&#123;&#123;</span> <span class="attr">...theme</span>, <span class="attr">changeBg</span>, <span class="attr">changeColor</span>, <span class="attr">changeSize</span> &#125;&#125;</span></span></span><br><span class="line"><span class="tag"><span class="language-xml">    &gt;</span></span></span><br><span class="line"><span class="language-xml">      <span class="tag">&lt;<span class="name">Container</span>&gt;</span><span class="tag">&lt;/<span class="name">Container</span>&gt;</span></span></span><br><span class="line"><span class="language-xml">    <span class="tag">&lt;/<span class="name">ThemeContext.Provider</span>&gt;</span></span></span><br><span class="line">  );</span><br><span class="line">&#125;</span><br><span class="line"><span class="keyword">function</span> <span class="title function_">Container</span>(<span class="params"></span>) &#123;</span><br><span class="line">  <span class="keyword">const</span> &#123; bg, changeBg, changeColor, changeSize &#125; = <span class="title function_">useContext</span>(<span class="title class_">ThemeContext</span>);</span><br><span class="line">  <span class="variable language_">console</span>.<span class="title function_">log</span>(<span class="string">&#x27;render Container&#x27;</span>);</span><br><span class="line">  <span class="keyword">return</span> (</span><br><span class="line">    <span class="language-xml"><span class="tag">&lt;<span class="name">div</span> <span class="attr">style</span>=<span class="string">&#123;&#123;</span> <span class="attr">padding:</span> <span class="attr">20</span>, <span class="attr">background:</span> <span class="attr">bg</span> &#125;&#125;&gt;</span></span></span><br><span class="line"><span class="language-xml">      <span class="tag">&lt;<span class="name">div</span>&gt;</span></span></span><br><span class="line"><span class="language-xml">        <span class="tag">&lt;<span class="name">button</span></span></span></span><br><span class="line"><span class="tag"><span class="language-xml">          <span class="attr">onClick</span>=<span class="string">&#123;()</span> =&gt;</span> &#123;</span></span><br><span class="line"><span class="language-xml">            changeBg(&#x27;gainsboro&#x27;);</span></span><br><span class="line"><span class="language-xml">          &#125;&#125;</span></span><br><span class="line"><span class="language-xml">        &gt;</span></span><br><span class="line"><span class="language-xml">          change bg</span></span><br><span class="line"><span class="language-xml">        <span class="tag">&lt;/<span class="name">button</span>&gt;</span></span></span><br><span class="line"><span class="language-xml">        <span class="tag">&lt;<span class="name">button</span> <span class="attr">onClick</span>=<span class="string">&#123;changeSize&#125;</span>&gt;</span>change size<span class="tag">&lt;/<span class="name">button</span>&gt;</span></span></span><br><span class="line"><span class="language-xml">        <span class="tag">&lt;<span class="name">button</span> <span class="attr">onClick</span>=<span class="string">&#123;changeColor&#125;</span>&gt;</span>change color<span class="tag">&lt;/<span class="name">button</span>&gt;</span></span></span><br><span class="line"><span class="language-xml">      <span class="tag">&lt;/<span class="name">div</span>&gt;</span></span></span><br><span class="line"><span class="language-xml">      <span class="tag">&lt;<span class="name">ParagraphMo</span></span></span></span><br><span class="line"><span class="tag"><span class="language-xml">        <span class="attr">text</span>=<span class="string">&#123;</span>&#x27;<span class="attr">Laugh</span> <span class="attr">and</span> <span class="attr">the</span> <span class="attr">world</span> <span class="attr">laughs</span> <span class="attr">with</span> <span class="attr">you</span>, <span class="attr">weep</span> <span class="attr">and</span> <span class="attr">you</span> <span class="attr">weep</span> <span class="attr">alone.</span>&#x27;&#125;</span></span></span><br><span class="line"><span class="tag"><span class="language-xml">      &gt;</span><span class="tag">&lt;/<span class="name">ParagraphMo</span>&gt;</span></span></span><br><span class="line"><span class="language-xml">      <span class="tag">&lt;<span class="name">Author</span> <span class="attr">author</span>=<span class="string">&#123;</span>&#x27;<span class="attr">someone</span>&#x27;&#125;&gt;</span><span class="tag">&lt;/<span class="name">Author</span>&gt;</span></span></span><br><span class="line"><span class="language-xml">    <span class="tag">&lt;/<span class="name">div</span>&gt;</span></span></span><br><span class="line">  );</span><br><span class="line">&#125;</span><br><span class="line"><span class="keyword">function</span> <span class="title function_">Paragraph</span>(<span class="params">&#123; text &#125;</span>) &#123;</span><br><span class="line">  <span class="variable language_">console</span>.<span class="title function_">log</span>(<span class="string">&#x27;render Paragraph&#x27;</span>);</span><br><span class="line">  <span class="keyword">const</span> theme = <span class="title function_">useContext</span>(<span class="title class_">ThemeContext</span>);</span><br><span class="line"></span><br><span class="line">  <span class="keyword">return</span> (</span><br><span class="line">    <span class="language-xml"><span class="tag">&lt;<span class="name">div</span>&gt;</span></span></span><br><span class="line"><span class="language-xml">      <span class="tag">&lt;<span class="name">p</span> <span class="attr">style</span>=<span class="string">&#123;&#123;</span> <span class="attr">color:</span> <span class="attr">theme.primary</span>, <span class="attr">fontSize:</span> <span class="attr">theme.fontSize</span> &#125;&#125;&gt;</span>&#123;text&#125;<span class="tag">&lt;/<span class="name">p</span>&gt;</span></span></span><br><span class="line"><span class="language-xml">    <span class="tag">&lt;/<span class="name">div</span>&gt;</span></span></span><br><span class="line">  );</span><br><span class="line">&#125;</span><br><span class="line"><span class="keyword">const</span> <span class="title class_">ParagraphMo</span> = <span class="title class_">React</span>.<span class="title function_">memo</span>(<span class="title class_">Paragraph</span>);</span><br><span class="line"><span class="keyword">function</span> <span class="title function_">Author</span>(<span class="params">&#123; author &#125;</span>) &#123;</span><br><span class="line">  <span class="variable language_">console</span>.<span class="title function_">log</span>(<span class="string">&#x27;render Author&#x27;</span>);</span><br><span class="line"></span><br><span class="line">  <span class="keyword">return</span> (</span><br><span class="line">    <span class="language-xml"><span class="tag">&lt;<span class="name">ThemeContext.Consumer</span>&gt;</span></span></span><br><span class="line"><span class="language-xml">      &#123;(theme) =&gt; &#123;</span></span><br><span class="line"><span class="language-xml">        return (</span></span><br><span class="line"><span class="language-xml">          <span class="tag">&lt;<span class="name">div</span> <span class="attr">style</span>=<span class="string">&#123;&#123;</span> <span class="attr">textAlign:</span> &#x27;<span class="attr">right</span>&#x27;, <span class="attr">color:</span> <span class="attr">theme.secondary</span> &#125;&#125;&gt;</span></span></span><br><span class="line"><span class="language-xml">            —— &#123;author&#125;</span></span><br><span class="line"><span class="language-xml">          <span class="tag">&lt;/<span class="name">div</span>&gt;</span></span></span><br><span class="line"><span class="language-xml">        );</span></span><br><span class="line"><span class="language-xml">      &#125;&#125;</span></span><br><span class="line"><span class="language-xml">    <span class="tag">&lt;/<span class="name">ThemeContext.Consumer</span>&gt;</span></span></span><br><span class="line">  );</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>点击按钮会分别改变对应的全局样式,效果如下：</p>
<p><img src="./../imgs/89ff069b244a4e5d8073f425cab0a975.png" alt="image.png"></p>
<p>Context+state+dispatch+reducer，看上去Redux的几个要素都有了，并且也实现了全局的状态管理。</p>
<p>但是当我们点击_change bg_按钮的时候，三个组件都触发了渲染：</p>
<p><img src="./../imgs/751b1d25cf79455ba6d7e3edcffeda52.png" alt="image.png"></p>
<p><strong>这就是Context的问题所在:不必要的渲染</strong>。即使没有依赖改变的值，也会被迫re-render。</p>
<h2 id="对Context的误解"><a href="#对Context的误解" class="headerlink" title="对Context的误解"></a>对Context的误解</h2><p>关于Context，有几个常见的误解：</p>
<ol>
<li>Context是用来做全局状态管理的</li>
</ol>
<p>事实上Context并不是解决全局状态管理，而是化解props传递的繁琐</p>
<blockquote>
<p>Context provides a way to pass data through the component tree without having to pass props down manually at every level.</p>
</blockquote>
<ol start="2">
<li>修改Context会触发组件的更新</li>
</ol>
<p>react对context value并没有监听变化，直接修改context value是无法触发更新的，真正触发组件更新的是setState。</p>
<ol start="3">
<li>当Context发生变化时，只有依赖到这些变化的属性的组件才会更新</li>
</ol>
<p>只要组件依赖了Context，不管变化的是那个局部属性，组件都会被强迫更新。即使加上了<code>memo</code>也没用用,因为<code>memo</code>比较的是props。</p>
<h2 id="如何基于Context设计全局状态管理"><a href="#如何基于Context设计全局状态管理" class="headerlink" title="如何基于Context设计全局状态管理"></a>如何基于Context设计全局状态管理</h2><p>如果仅基于hooks的话，可以参考这篇<a href="https://blog.axlight.com%2Fposts%2Ffour-patterns-for-global-state-with-react-hooks-context-or-redux%2F" title="https://blog.axlight.com/posts/four-patterns-for-global-state-with-react-hooks-context-or-redux/">文章</a><a href="https://blog.axlight.com%2Fposts%2Ffour-patterns-for-global-state-with-react-hooks-context-or-redux%2F" title="https://blog.axlight.com/posts/four-patterns-for-global-state-with-react-hooks-context-or-redux/">blog.axlight.com&#x2F;posts&#x2F;four-…</a>.</p>
<p>不过react-redux已经很好地回答了这个问题，其中的一个关键就是Provider中创建的subscription。</p>
<p>最后总结一下：</p>
<blockquote>
<p>You Might Not Need Redux (But You Can’t Replace It With Hooks)</p>
</blockquote>

        
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